In recent years, as a technique for increasing communication capacity, Multi-User Multiple-Input Multiple-Output (MU-MIMO) schemes are known, for example, by which data signals intended for a plurality of terminal devices (represented by pieces of User Equipment (UEs)) are spatially multiplexed by using mutually the same wireless resource. A base station calculates transmission weights by using a method such as, for example, a Zero-Forcing method or a Minimum Mean Square Error (MMSE) method, while using channel information between the UEs that are subject to the spatial multiplexing process. Further, by forming beams by multiplying the data signals by the calculated transmission weights, the base station is able to reduce signal interference between the UEs.
FIG. 13 is a drawing for explaining an example of a cause of degradation of a Signal to Interference Noise Ratio (SINR) between a base station and UEs. In the wireless system illustrated in FIG. 13, there is a discrepancy between ideal channel information H used for calculating the transmission weights and actual channel information H′. As the discrepancy becomes larger, signal interference between the UEs also increases, and the SINR thereof is also degraded. Further, as the SINR is degraded, demodulation capability for the data signals is also degraded. In addition, for example, also when correlation of channels between UEs becomes higher as a result of the UEs being positioned closer to each other, degradation of the SINR becomes significant.
In a wireless system using a Long Term Evolution (LTE) scheme or a New Radio (NR) scheme, for example, UEs demodulate data signals by using DeModulation Reference Signals (DMRSs) received from a base station. In that situation, the base station multiplies the DMRSs by the same transmission weights as those used for multiplying the data signals and further transmits the multiplied DMRSs to the UEs.
Each of the UEs refers to the DMRS received from the base station and estimates channel information about a channel used for propagating the data signal. Further, each of the UEs demodulates the data signal by using the estimated channel information. However, at each of the UEs, as the SINR becomes degraded, the level of precision for estimating the channel information also becomes degraded. As a result, the demodulation capability for the data signals is also degraded.
To cope with this situation, according to a NR scheme, for example, orthogonal DMRS ports (hereinafter, simply “orthogonal ports”) which are orthogonal to one another and of which the maximum quantity is 12 are defined for the purpose of improving the level of precision for the estimation of the channel information made by the UEs. In this situation, because the orthogonal ports use mutually-different time periods, frequencies, or codes, even when there is a discrepancy (or an error) in the channel information used for calculating the transmission weights, no signal interference will occur among the UEs. However, for example, when thirteen or more UEs are subject to the spatial multiplexing process, using only the orthogonal ports is not sufficient. In that situation, non-orthogonal DMRS ports (hereinafter, simply “non-orthogonal ports”) will be adopted by which signal sequences of the DMRSs are changed by using scramble IDs or virtual cell IDs, while using the same time period, frequency, or code with the orthogonal ports. As a result, for example, even when the spatial multiplexing process is to be performed on UEs of which the quantity exceeds the number of orthogonal ports, it is possible to reduce signal interference among the UEs.    Patent Literature 1: Japanese Laid-open Patent Publication No. 2014-131202
However, at the base station, even when non-orthogonal ports are assigned to one or more of the UEs, because orthogonalization of the DMRS ports is not perfect, there may be a discrepancy (an error) in the channel information used for calculating the transmission weights, which may lead to signal interference among the UEs. In other words, the base station is not able to assign an optimal DMRS port, i.e., an optimal reference signal to each of the UEs. As a result, as the signal interference between UEs becomes larger, the SINRs between the two become degraded. Accordingly, the demodulation capability for the data signals is also degraded.